1. Field of the Invention
The invention is directed generally to printed circuit boards and more specifically to a multichip module for surface mounting on printed circuit boards, having electronic components fixed on a thin-film or thick-film substrate and having SMD-compatible terminal elements.
2. Description of the Related Art
The goal of providing the entire circuitry of an electronic assembly having a plurality of ICs in a single component without the complications of providing application-tailored ICs can be currently achieved with a multichip module (MCM). With an MCM, thin-film or thick-film substrates are utilized as carriers of standard components, particularly for commercially-obtainable ICs that are connected with one-layer or multi-layer circuit structures applied on the substrate. Housed or unhoused, such MCMs should be capable of being further-processed like discrete components, i.e. capable of being integrated in other systems. Thin-film substrates are manufactured of high-quality ceramic or glass and have resistive layers, dielectric layers and interconnect layers deposited in vacuum technologies and subsequently photo-technically structured. Compared to thick-film circuits manufactured in silkscreening processes, such thin-film substrates are in fact distinguished by higher quality features (wiring density, precision and stability of the components integrated into the thin-film circuit structures), but are relatively cost-intensive dependent on the substrate area. Given the same area, the costs of a thin-film substrate are a multiple of the costs of a corresponding thick-film circuit.
In previous multichip modules provided for integration in printed circuit boards, the terminal elements are attached to the ceramic substrate. To compensate for the different thermal expansions between the substrate ceramic and the printed circuit board, terminal combs, generally standard in hybrid technology, are used. The overall area of the module, however, is enlarged as a result thereof, since the terminal combs project above the substrate surface. In addition, the number of possible terminals is too low for many applications. Moreover, the relatively cost-intensive terminal combs can be easily damaged during handling.
International application WO 92/11654 discloses bonding an individual chip to a printed circuit board instead of a ceramic lamina for cost reasons and to improve the electrical values. In view of a specific re-housing method, in particular, it is also proposed therein that the printed circuit board carrying the chip not be impressed with plug-in pins into corresponding receptacle holes of a main board as usual but that accumulations of solder capable of reflow be provided on the backside of the through-contacted carrier printed circuit board. The chip is preferably directly secured on the printed circuit board; however, it can also be fixed with a conductive adhesive. In order to resolve the problem of a different thermal expansion between a printed circuit board substrate of plastic and the silicon chip having increasing size of the discrete chip, it is also proposed to provide a metallic reinforcing layer between the conductive adhesive of the chip and the interconnects of the carrier printed circuit board. The metallic reinforcing layer reduces the mechanical pressure forces. However, multichip modules are not mentioned in the application.